The present invention relates to a method for producing interleukin-2.
Interleukin-2 [hereinafter abbreviated IL-2, which is also called T-cell growth factor (TCGF)] is a lymphokine produced by T-cells stimulated by lectin, allotype antigens, etc. [Science, Vol. 193, p. 1007 (1976)].
Using IL-2, a great number of clones of killer T-cells, helper T-cells, natural killer cells etc. have been obtained [e.g. Nature, Vol. 268, p. 154 (1977)]. In addition to such killer cells, IL-2 can be used in the preferential proliferation in vitro of antigen-specific killer T-cells, which recognize and destroy specific antigens such as tumor antigens. It is also possible to inhibit tumor growth by transferring thus proliferated tumor-specific killer T-cells into animals [The Journal of Immunology, Vol. 125, p. 1904 (1980)].
These experimental facts suggest the great potential for the application of IL-2 as an antitumor agent. It is also known that IL-2 promoted recovery of the helper T-cell function in nude mice lacking thymic function [European Journal of Immunology, Vol. 10, p. 719 (1980)] and recovery of the induction of killer T-cells to said helper T-cells [Nature, Vol. 284, p. 278 (1980)]; it is expected that IL-2 will also be applied in the therapy of immunodeficiency diseases.
Human IL-2 can be obtained from human T-cells, but only in extremely small quantities. Owing to the recent progress of gene recombination technology, however, it has become possible to obtain human IL-2 as a bioactive protein from a culture of Escherichia coli, of Escherichia coli etc. possessing expression vectors to which a human IL-2 gene has been transferred [Nature, Vol. 302, p. 305 (1983); Nucleic Acids Research, Vol. 11, p. 4307 (1983)].
Conventional human IL-2 production methods are not generally favorable for industrial applications because of their low human IL-2 productibility.
In view of this, the present inventors studied the cultivation methods for E. coli possessing IL-2 productibility, finding that such productibility is considerably improved by culturing E. coli under acidic pH conditions of from about 4.8 to 6, this despite the fact that fermentation of E. coli had typically been carried out under neutral pH conditions of approx. from 6.5 to 7.5 as it was generally regarded that neutral or slightly alkaline conditions were preferred [Biochemical Engineering, University of Tokyo Press, 25-26 (1965)].
Based on this finding, the inventors made further studies, developing the present invention.